Optical Coherence Tomography (OCT) is a technology for performing high-resolution cross sectional imaging that can provide images of tissue structure on the micron scale in situ and in real time. OCT is a method of interferometry that uses light containing a range of optical frequencies to determine the scattering profile of a sample. The axial resolution of OCT is inversely proportional to the span of optical frequencies used.
In recent years, it has been demonstrated that frequency domain OCT (FD-OCT) has significant advantages in speed and signal to noise ratio as compared to time domain OCT (TD-OCT; Leitgeb et al. 2003; deBoer et al. 2003; Choma et al. 2003).
In FD-OCT, a light source capable of emitting a range of optical frequencies enters an interferometer, the interferometer combines the light returned from a sample with the light from a reference arm, and the intensity of the combined light is recorded as a function of optical frequency to form an interference spectrum. A Fourier transform of the interference spectrum provides the reflectance distribution along the depth within the sample. FD-OCT requires some means to record the interference spectrum, the intensity of light output from the interferometer as a function of optical frequency. Current methods of FD-OCT can be divided into two categories.
In spectral-domain OCT (SD-OCT), a grating or prism or other means is used to disperse the output of the interferometer into its optical frequency components. The intensities of these separated components are measured using an array of optical detectors, each detector receiving an optical frequency or a fractional range of optical frequencies. The set of measurements from these optical detectors forms an interference spectrum (Smith & Dobson 1989). Typically the light source emits a broad range of optical frequencies simultaneously.
Alternatively, in swept-source OCT (SS-OCT), the interference spectrum is recorded by using a source with adjustable optical frequency, with the optical frequency of the source swept through a range of optical frequencies, and recording the interfered light intensity as a function of time during the sweep (U.S. Pat. No. 5,321,501). FD-OCT efficiently uses the light returned from a range of depths within the sample, as all the returned light contributes to the modulation in the interference spectrum.
FD-OCT methods use the fact that interference between light scattered from the sample and the reference beam causes spectral interference fringes, a modulation in the intensity of the combined beam as a function of optical frequency. The spacing of the interference fringes depends on the difference in optical group delay between the light scattered from the sample, and reference light.
OCT determines the position in the sample of a scattering center based on the difference in optical group delay between two optical paths: 1) the path of light scattered from the sample, and 2) a reference optical path.
Most OCT literature refers simply to the difference between sample and reference optical path lengths. However, due to the dispersive nature of not only transmissive optics (such as fibers), but the sample itself, there will be spectral differences introduced between these two optical path lengths, thus effecting the measured signal and its interpretation. Most transmissive optical components generate positive chromatic dispersion. Alternatively, there are some optical components, such as gratings, where the chromatic dispersion is negative.
Chromatic dispersion is a property of an optical element that characterizes the degree by which the optical path length through that element varies across a range of optical frequencies. Chromatic dispersion is the result of wavelength components travelling at different phase and/or group velocities in a dispersive medium. Without proper compensation, the phase of the signal varies causing degradation in axial resolution. Due to the nature of most dispersive materials, blue wavelengths of light travel slower in a medium than their red counterparts. Thus a pulse of polychromatic light will be broadened temporally. This temporal shape modification is a function of the degree of second-order (and higher terms as well) in the functional form for chromatic dispersion (discussed in detail hereinbelow).
In the case of chromatic dispersion, there needs to be a distinction between the phase delay and the group delay associated with a given optical path length. OCT is sensitive to the difference in group delay (see, for example, section 2.1 in Fercher et al. 2001). FD-OCT necessarily uses a range of optical frequencies. If the chromatic dispersion is not matched between the two paths, the apparent position of the scattering center depends on the optical frequency used. A mismatch in chromatic dispersion thus broadens the axial resolution of the OCT as explained by Hitzenberger et al. 1999. For this reason, in most FD-OCT systems the chromatic dispersion is closely matched or at least adjusted between sample and reference paths (see, e.g., U.S. Pat. Nos. 6,385,358, 6,615,072, 6,618,152) sometimes through the use of dispersive optical devices (see, e.g., U.S. Pat. No. 6,282,011). Since a perfect match of chromatic dispersion is not simple, it can add cost to the system by requiring tight optical tolerances on the various optical components.